Liquid densitometers



July 5, 1960 Filed Sept. 3, 1957 H. A. BERNSTEIN LIQUID DENSITOMETERS 3Sheets-Sheet 1 Jul? 5, 1960 H. A. BERNSTEIN 2,943,476

LIQUID DENSITOMETERS Filed sept. 5, 195'? s sheets-sheet 2 fw we f ya??@d l f f /W/ /f/ mvtNToR July 5, 1960 H. A. BERNSTEIN 2,943,476

LIQUID DENSITOMETERS Filed Sept. 3, 1957 3 Sheets-Sheet 3 J k J i L D C3 @y L) (3 L /C a D C) 3? c Q F D D w r r LIQUID DEN SIT OWTERS HaroldA. Bernstein, Cardiff, Wales, assignor to Simmonds AerocessoriesLimited, Glamorganshire, Wales Filed sept. s, i957, ser. No. 681,545

a Claims. (ci. 733z This application is a continuation-in-part of my application Serial No. 557,573, filed January 5, 1956, now abandoned.

This invention relates to liquid densitometers, and more particularly todensitometers capable of providing a continuous signal dependent on thedensity of a liquid which may be moving or stationary.

AIt is often desirable to have an indication of liquid volume or theamount or rate of liquid ow in terms of mass.

The power which can be obtained Ifrom an aircraft fuel, for example,depends not on its volume but on its mass, and it is thus desirable tohave the amount of fuel available, and its rate of consumption,indicated continuously in terms of mass. Moreover an accurate knowledgeof fuel mass taken on by an aircraft is of great importance to itsoperator. A long-range aircraft may have fuel tank capacity of 7000limperial gallons; now AVTUR commercial grade fuel, for example, may at agiven temperature vary in specific gravity between 0.78 and 0.83representing a weight of Iabout 11/2 tons for the volume considered. lfthe mass of fuel taken on can be accurately determined, the aircraftpayload can be greater since the allowance for possible errors in masscan be reduced.

It is, therefore, a main object of the invention to provide a liquiddensitometer which can conveniently be combined with a liquid contentgauge or flow meter of the kind sensitive to liquid volume whereby therequired continuous indication in terms of mass can be obtained.

A densitometer o-f this kind will of course register -density changesdue to temperature; this overcomes various diiculties associated withdensity-correcting devices of the kind which are manually set inaccordance with occasional density measurements. It is naturallyimportant that temperature variations should not make the densitometerinaccurate and it is a subsidiary object of the invention to provide adensitometer the accuracy of which is unaected by temperature change andwhich furthermore does not require for this purpose complicatedmechanical or electrical arrangements.

The liquid densitometer according to the invention comprises a support,a hollow member mounted for vibration with respect to the support on oneor more supporting elements resilient at least in part and carried bythe support remote from the connection o-f the element(s) with thehollow member, the hollow member being such as not to flex appreciablyon vibration, means enabling the continuous passage of liquid into andout of the hollow member said member being completely full of liquidduring operation of the densitometer, means for vibrating the hollowmember with respect to the support at the natural frequency ofvibration, and means to provide an electric signal having acharacteristic dependent on the frequency of said vibration and therebyon the density of the liquid.

3,476 Patented July 5, 1960 Preferably the vibrating means is electricaland comprises one or more electrical elements which form, or form partof, said signal-providing means and are adapted to provide a signalhaving a frequency proportional to the frequency of vibration.

In one preferred form of densitometer the resilient elements are Va pairof equal tubes providing the means enabling the passage of liquid to thehollow member and supporting it in the manner of cantilevers. Accordingto an important feature of the invention the two tubes are made of 4ametal which has such a composition, and is so treated, that its modulusof elasticity changes with temperature in the opposite sense (as regardsthe density-dependent signal) to changes with temperature in the`dimensions of the densitometer, whereby, despite said dimensionchanges, signals provided by the densitometer are substantiallyindependent of ambient temperature over a working range.

Six examples of densitometer according to the invention will now bedescribed with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic plan view of the first densitometer;

1rigure 2 is a diagrammatic side elevation of the rst densitometer;

Figure 3 is a diagram of an oscillator circuit forming part of the rstdensitometer;

Figure 4 is a perspective view of the first densitometer showing onepractical form it may take, and

Figures 5 to 9 are diagrammatic plan views of the second, third, fourth,fifth and sixth densitometers respectively.

ln the iirst densitometer (see Figures 1 and 2), a hollow cube 1 made ofsoft magnetic material is rigidly secured to a pair of resilient tubes2, 3 extending perpendicularly from a vertical side face of the cube atpositions near the mid-points of the upper and lower edges respectivelyof this face. The tubes extend through a clamping block 4 secured to arelatively massive support (not shown) and are arranged for connectionto a liquid line (not shown) e.g. by means of bleeds, so that liquid iscontinuously circulated through one pipe 2, into the cube 1, and outthrough the other pipe 3, the cube being always completely full ofliquid during operation of the densitometer.

A pair of U-pieces 5, 6 of a soft magnetic material is arranged oneither side of the cube 1 in a horizontal plane with the free ends oftheir limbs 7, 8i, 9, 10i, directed towards the vertical faces of thecube perpendicular to the face from which the tubes 2, 3 extend, andseparated yfrom such faces by an air gap. A magnetic circuit is thusestablished through each U-piece 5, 6 and through the adjacent part ofthe cube 1. Driving coils 11, 12 are provided one on one limb '7, 9 ofeach U-piece 5, 6 and the coils are connected to form part of anoscillator.

The `oscillator may be such as shown in Figure 3, which iselectro-mechanical in operation, the feedback which produces theoscillation resulting Ifrom variation of the magnetic circuits formed bythe coils 1'1, 12 and the cube 1 on the movement of the latter. TheFigure 3 `oscillator comprises a tr-iode valve 14 with its anode 15connected to an H.T. line i6 through one of the coils 11 and its cathode17 connected tov an earth line 18 through ya capacitor 19 :and variableresistor Ztl in parallel. The setting of the resistor Ztl controls theanode current in the valve 14 and hence the amplitude of the movement ofthe cube 1. The grid 21 of the valve `14 is connected to the earth line18 through a high resistance 22, and, through a capacitor 23, to thejunction point between the other coil 12 and a resistor 24, this coil 12and resistor 24 being in series between the H.T. and

earth lines 16 and 1S. A signal is taken from any convenient point inthe circuit e.g. the anode 15 of the valve 14.

As will be appreciated, the valve 14 maintains the mechanicaloscillation of the cube 1 and `the natural frequency of the vibration ofthe cube kdetermines the fren quency of the electrical signals, which donot require separate adjustment to be equal to that frequency.

flfhe cube .1 is caused to vibrate at its natural 1frequency between theU-pieces S, 6 ie. transverse to the tubes 2, 3, which as will beappreciated also vibrate, their vibration kbeing that of loadedcantilevers. The signal from the oscillator, the rfrequency of whichis'that of the vibration, is made use of as desired.

Thefrequency of vibration ofthe cubef'lis given'by:

where:

E is the elastic modulus of the tubes,

I`is the moment of inertia of the tubes `(or rather of such portions asare free to vibrate),

M is the mass of the cube, Y

k is the radius of gyration of the cube about an axis perpendicular tothe tubes and to the plane of'vibration,

M1 is the mass of the tubes,

L is the free length of the tubes, and

l is the distance between thecentre of gravity of the cube `and thesupport.

The mass of the cube includes themass of liquid it contains and the`expression given above accordingly relates frequency of vibration tothe density of the liquid.

Figure 4 shows a practical form of the first densitometer suitable formass-production. Parts corresponding to those of the more diagrammaticFigures l and2 are given the same numerals distinguished by a prime. The.chief differences of appearance between Ylligures'l and 2 and Figure 4are due to the fact that in Figure 4 the U-pieces =5'Y and 6 'arecarried by a relatively massive andrigid yoke200 providing the supportfor lthe tubes 2.',.3.

The yoke 200 is made of non-magnetic stainless steel nor preferably ofaluminum alloy for the sake of lightness; the limbs 7', 8-9, 10' of the`U-pieces 5', 6' are rigidly screwed to the arms 201, 202 of the yokeandinterconnected by cross members 203, r204 form- :iug the bights ofthe Us. An integral connectingpiece '205 Aextends between theends of thearms 201, 202 to rigidify the yoke 200. The hollow member 1' is formedby a'cylinder of magnetic stainless steel which has a. diameter only alittle smaller than the spacing of the opposed pole pieces provided bythe limb of the U-pieces 55', 6'; the cylinder is closed at its ends andhas its axis V(when it is at rest) coincident with the centre line ofthe yoke 200. Each of the two tubes Z', 3' carrying the hollow member 1'is silver-soldered to a brass sleeve 206 rigid with the base of the yoke200; the general plane of the tubes contains the yoke centre line. Thetubes 2', 3' are made of a heat-treated nickel-chromium- `Vtitanium-ironalloy, such as the alloy Ni-Span C obtainable from Henry Wiggin &Company Limited.

The alloy NiSpan C has the following general com- `position:

' Carbon e.. Up to .06%. Silicon .'Up to 1.0%. i Phosphorous Upto .04%.LSulphur Upto .04%. Manganese v--. Up to 0.8% "fChromium 4.9-'5.5%.Nickeland cobalt 1l-43%.

(Cobalt alone up to 1.0%

4 Titanium 2.2-2.6%. Aluminium 0.3-0.8%. Iron The remainder.

' The alloy Ni-Span Ccan, however, -bemade to give a `oscillator asdescribed above.

zero or positive coefficient though it has heretofore been employed onlyfor springs in precision instruments and in analogous uses where aconstant modulus of elasticity is desirable. The densitometer accordingto the invention makes use of the fact that the alloy can be given apositive coecient, and doesso in the following way.

On rise of temperature all the parts of the-.densitometer expand. Theterms of the above equation relating the frequency of natural vvibrationto the density of liquid being passed Athrough the densitometer changecorrespondingly, the effective length ofthe tubes 2.', 3 increasingtogether` with the radius of gyration .of the hollow member '1',and thevolumeofthis member also increasing. Another effect of expansion is toincrease the air gaps between the pole pieces 7', .8', 9', 10' and thecylinder 1' and hence reducethe strength of magnetic iield; this tendsto reduce the frequency of vibration, other factors remaining the same.

However the material of the `tubes is so treated that its modulus ofelasticity increases with temperature in a-manner to compensate for the.changes with tempera` `ture mentioned inthe foregoingparagraph.

The densitometer just described isparticularly suitable for use inaircraft, as it can be made small and light (eg. under 2 lbs), isaffected by vacceleration to a negligible degree, and can be used overthe wide range of temperature often encountered without givinginaccurate indications or requiring complicated correcting circuits.

The second densitometer is illustrated inFigureS and the hollow memberis ahollow cube 30 of soft magnetic material rigidly connected to a pairof resilient tubes 31, 32 extending coaxially from opposite sides of thecube. The tubes 31, 32 extend through rigid clamping blocks 33, 34respectively, their free lengths being equal,

-and are arranged for liquid ow lthrough one tube 31,

through-the cube 30, and out through the other tube 32.

The cube 30 (and the tubes 31, 32) .are caused to vibrate by means ofU-pieces 35, 36, coils 37, 38 and an The frequency of natural vibrationof ther cube 30 and tubes 31, 32, which act similarly -tofa loaded-beamencastered at its ends, is; given 1 195 111g' 21T Lulu-toenam) `firstdensitometer, but itis distinguished therefrom by vibration of the cubetaking place parallel to the length of the tubes 102, 103, by reasonv ofalternating axial stress therein. To effect such-vibration a U-piece ofa. soft magnetic material is arranged closeto the face of the cube 101remote from the face from which thetubes extend, the limbs 106, 107 ofthe U-piece being aligned with the tubes 102, 103 .to minimize vibrationof the wall of the cube in the manner of a diaphragm. Bach limb 106, 107carries a driving coil 108, 109 similar to the coils 11 and 12; thecoils 108, 109 are connected in an loscillator circuit similar to thatof Figure 3.

The fourth densitometer, illustrated in Figure 7 differs from that ofFigure 6 only in the driving arrangement and parts similar to those ofFigure 6 are given the same reference numerals distinguished by a prime.The driving arrangement comprises a pair of permanently magnetic screws110, 111 adjustably mounted in opposite arms of a yoke (not shown) ofsoft magnetic material whereby one screw 110 is opposite the mid-pointof and perpendicular to the face of the cube 101' from which the tubes102', 103 extend, and the other screw 111 is similarly placed withrespect to the opposite face of the cube. Driving coils 112, 113 aremounted one on each screw and connected in an oscillator circuit such asshown in Figure 3. A magnetic circuit is established through the yoke,screw 110, cube 107', screw 111 and back to the yoke, and vibrationtakes place parallel to the tubes 102', 103'. Adjustment of the screwsin the yoke permits variation of the driving force. The faces of thecube 101 through which the driving forces are transmitted arecomparatively rigid to minimize their Vibration as diaphragms.

The fifth densitometer illustrated in Figure 8 comprises -a hollow cube121 similar to the cube 101 of Figure 6 and carried by `a pair lofparallel tubes 122, 123 similar to the tubes 102, 103. These tubes 122,123 are not, however, rigidly fixed to the massive support 124; a pairof cylindrical recesses 125, 126 are formed in the support in alignmentwith the tubes and with bores 127, 128 whereby liquid is supplied to thecube 121 in the manner indicated in the ligure. Each recess 125, 126receives a comparatively thick-Walled cylindrical plug of solder 129,130 which in turn receives the end of one tube 122, 123 remote from thecube 121, the plugs being connected by soldering to the support and tothe tubes. Either of the driving arrangements described with referenceto Figures 6 and 7 may be used for Vibrating the cube 121 parallel tothe tubes.

The sixth densitometer illustrated in Figure 9 com prises a hollow cube1131 made of soft magnetic material; a pair of similar parallel seamlessbellows 132, 133 each have one end connected to the cube and the otherend connected to a rigid massive support 134. Passages (not shown) areprovided in the support 134 whereby liquid can be circulated throughinlet 135, bellows 132, cube 131, bellows 133 and outlet 136. Either ofthe driving arrangements of Figures 6 and 7 can be used to vibrate thecube parallel to the bellows.

In the rst and second densitometers vibration takes place transverselywith respect to the resilient means supporting the hollow member and canhave a frequency as low as a few cycles per second; in all thedensitometers of Figures 6 to 9 the vibration takes place longitudinallyof such members and the frequency of vibration will generally be highespecially in the third and fourth densitometers where the `frequencydepends on the modulus of elasticity `of the material forming the tubes102, 103 or 102', 103. In the fth densitometer, however, thecomparatively low modulus of elasticity of the solder reduces thefrequency as to some extent the whole assembly of tubes 122, 123 andcube 121 vibrates in the solder plugs 129, 130. In the sixthdensitometer the use of bellows instead of tubes gives a lower frequencyof vibration than is the case with the third and fourth densitometers,with comparable materials and dimensions.

It will be appreciated that many variations of the third, fourth, iifthand sixth densitometers are possible. Thus the two tubes 102, 103 of thelilith densitometer may be replaced by a single symmetrically-arrangedtube, a sec- 6 ond tube being disposed within the first to extend Wellinto the cube whereby liquid is introduced into the cube through theannular space between the tubes and removed through the second tube.

The bellows 132, 133 of the sixth densitometer can be replaced by coilsprings, the liquid being conveyed to and from the cube 131 by exibletubes.

It Will be understood that though, in the densitometers just described,a cube has been mentioned as the main liquid-containing member it is notnecessary that such member be cubical in shape, and that it may have anyother desired shape. The cube (or other-shaped hollow member) need notbe made of soft magnetic material; it may, for example be made ofnon-magnetic material and carry armatures of soft magnetic material tocoact with the electrical driving means causing vibration.

The driving means must, of course, provide a closed magnetic circuit(apant from inevitable air gaps) but can include E-pieces or pots asIwell as the U-pieces described above.

The U-pieces 5, 6 or 105, may, instead of being made throughout of softmagnetic material, comprise a portion which is permanently magnetic. InFigure 1 the limbs 8 and 10 may, for example, be permanent magnets therest of the U-pieces being made of soft magnetic material; alternativelythe bight portions of the U-pieces may be permanently magnetic. Wherethe U-pieces comprise permanently magnetic portions the circuit shown inFigure 3 should be modified to exclude D.C. components in the currentfed to the driving coils 11, 12. The arrangement then has the advantagesthat the current consumption is reduced and the pull of the U-pieces isnot affected by variations in the circuit.

ln addition, it is not necessary that the support be massive; if it isnot so, however, it should be isolated from the surroundings byvibration mounts.

The method of overcoming the effect of temperature on the densitometerdescribed in detail with reference to Figure 4 can be applied to theother forms of densitometer described.

In each densitometer described above, the signal from the membercontaining the liquid the density of which is to be measured, can becombined with a frequency provided by a fixed oscillator in a frequencyselective network so as to give an output having either the ratio, sumor difference of the frequencies of the input signals. This output maybe fed to a device comprising a meter sensitive to the volume of liquidflow whereby the device registers the amount or rate of liquid liow interms of mass. In a modified arrangement, a iixed D.C. level can be usedinstead of a fixed frequency.

Errors due to temperature variation can be reduced by using materialshaving low coeicients of expansion and elasticity.

It will be appreciated that, except with low frequencies of vibration,densitometers according to the invention will be little affected bytheir attitude and by accelerations such as may be encountered inaircraft, for example.

I claim:

l. A liquid densitometer comprising a support, a substantiallynondeformable hollow member, at least one supporting element resilientat least in part and connected at spaced points to the support and tothe hollow member, said supporting element being composed of an alloycontaining iron, chromium, nickel and titanium and having a modulus ofelasticity rising with temperature increase, means maintaining acontinuous iiow of liquid into and out of the hollow member where-by tokeep said member iilled with liquid during operation of the`densitometer, means imparting to the supporting element periodicallyvarying forces to cause the hollow member to vibrate with respect to thesupport at the natural frequency of vibration and means responsive tovibrationy of said hollow member for producing an electrical signalhaving a characteristic dependent on the frequency of said vibration and.thereby on the density of the liquid.

`2. A-densitometer as claimed `in claim 1, comprising two supportingelements,in thepformofl -apair of! equail bellows.

3. A densitometer as claimed in claim i1 comprising two supportingelements in Athe `form of equal parallel tubes.

4. A densitometer as claimed in claim lcomprising two supportingelements in the form of equal `parallel tubes each connected to thesupport through .aplug Vof solder which is vthereby 4set in vibration:on vibrationof the hollowmember.

5. A liquid densitometer comprising a support, a substantiallynondeformable hollow member, atleast one resilient tube rigid atspacedpoints withthesupport and with the hollow member to form Vacantilever mounting for the hollow member and to'form a liquid passagebetween the support an'dsaid member, means `providing asecondliquid'gpassagebetween the supportand said hollow member'to enablecontinuous circulation of liquid through said hollow member, the hollowmember being completely' full of liquid during operationro'f thedensitometer, means'to vibrate the hollow member with respect to thesupport at the natural frequency of vibration whereby to set upperiodically varyingloads on the cantilever which are transverse'to andintersect lthe axis thereof, and means responsive to vibration of saidhollow member for producing an electric signal having a characteristicdependent onthe density lof the liquid, said tube being composed of ametal having a modulus of elasticity changing with temperature in theopposite sense (as regards the density dependent signal) to dimensionalchanges of the densitometer whereby to minimize the effect of ambienttemperature on said density dependent signals.

6. A liquiddensitometer according to claim'S, wherein there are twosupporting elements in the form of hollow resilient tubes, one to' leadliquid to the-hollow member and one to lead liquid out of-suchmember,the tubes being aligned and'connected'to cpposite'sides of the hollowmember.

7. A liquid densitometer as claimed in claim "5, comprising a pair ofhollow resilient tubes, one to lead liquid to the hollow member and theother to'lead liquid out of such member (and thereby formthe'secondpassage providing means), the tub'being equal and parallel, the hollowmember comprising ferromagnetic material and the vibrating meanscomprising core mounted driving coils fixed relatively to the supportsymmetrically on either side lof the hollow member to formvariable-air-gap magnetic circuits therewith, each said tube being madeof an alloy containingchromium in the yregion of 5%, nickel in theregion of 40% and titanium in the region of 2.4% Vthe balance beingironand impurities, the alloy beingheattreated toihave rising modulus orelasticity with temperature `increase whereby to compensate `for theeiectp'f temperature-induced dimensional changes. on `said .8. A liquiddensitometer comprising arigid yoke, equal resilient inlet and outlettubes encastered inand extending through thebight of the kyoke withtheir axeslyingin the medial yplane between the limbs of said yoke, ahollow member symmetrically and rigidly mounted on the VVends of thetubes Within the yoke the tubes enabling continuous liquid circulationinto and out of said member, .coil-mounting cores rigidly secured to thelimbs of the yoke opposite thehollow member and providing therewith a;pair of magneticcircuits having air-gaps which vary inLlength onvibration of thehollow member transverse .to .said medial planethetubesbeing made ofa material'exhibiting a rise of modulus of elasticitywithrise-of temperature whereby to neutralize the eiect Vofdimensionalchanges due to change of ambient temperature, as regards thefrequency of vibration of the hollow member for a given liquid density,and means associated with said magnetic circuits to produce adensity-dependent electrical signal.

9. A liquiddensitometeras claimed in claim 8, whereinthetubes are madeof a metal having the following composition Carbon Up to .06%. SiliconUp to'1.07%. Sulphur Up to .04%. Phosphorous Up to -.04\%. Manganese Upto-0.8%. Chromium 4.9-5.5%. Nickel.and.-cobalt lll-43%.

V(Cobalt .alone upto 1.0% .l Titanium 2.2-2.6%. Aluminium 03.08%. IronThe remainder.

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